As a conventional device having a power supply device, a passenger detection electronic control unit (ECU) is disclosed in, for example, JP 2005-231411 A.
This passenger detection ECU is mounted on a vehicle to determine states of a passenger in a vehicle based on a detection result of a load sensor provided on a seat in a vehicle compartment. The passenger detection ECU includes a central processing unit (CPU) and a 5V-power supply circuit. The 5V-power supply circuit converts a voltage supplied from a vehicle battery to 5V and supplies it to the load sensor and the CPU. The CPU determines the state of the passenger in the vehicle based on the detection result of the load sensor. When the 5V-power supply circuit is in an abnormal state, the load sensor and the CPU do not operate normally. For this reason, the CPU monitors an output voltage of the 5V-power supply circuit and determines an operation state of the 5V-power supply circuit based on a monitored output voltage. For example, when the output voltage of the 5V-power supply circuit is in a normal output voltage range, the CPU determines that the 5V-power supply circuit is normal. When the output voltage of the 5V-power supply circuit is in a range other than the normal output voltage range, the CPU determines that the 5V-power supply circuit is abnormal.
The output voltage of the vehicle battery varies with a charge state and a load state. In a case that the 5V-power supply circuit is a regulated power supply circuit, the output voltage of the power supply circuit is maintained at a constant voltage even when the input voltage supplied from the vehicle battery varies. For this reason, as far as the 5V-power supply circuit is normal, its output voltage is maintained within the normal output voltage range and is not determined to be abnormal. In a case that the 5V-power supply circuit is not the regulated power supply circuit, the output voltage of the power supply circuit varies with a variation of the input voltage supplied from the vehicle battery. As a result, although the 5V-power supply circuit is normal, its output voltage is likely to vary to be outside the normal output voltage range and be determined erroneously to be abnormal.